The present invention relates to a combustion chamber, which is formed inside a diesel engine, defined by an inner surface of a cylinder, a crown surface of a piston, and a bottom surface of a cylinder head, and for being injected with fuel therein, the piston reciprocatable within the cylinder, the cylinder extending along a predetermined central axis, and the cylinder head opposing the crown surface of the piston.
Conventionally, in order to stimulate mixing of fuel with air inside diesel engines where the fuel is directly injected into a combustion chamber, a diesel engine is formed with a cavity in a crown surface of a piston thereof to concave in a direction away from a cylinder head so that a vertical flow of the fuel spray along a wall surface of the cavity (i.e., a tumble flow) is formed by a fuel injection.
For example, JP2010-121483A discloses a combustion chamber structure in which a cavity is formed in a central part of a crown surface of a piston. The cavity is formed to be a so-called reentrant cavity (i.e., a cavity bulging at its central ridge portion and tapering upward at its opening portion), and a fuel injection device is attached to the combustion chamber so that fuel spray is injected toward a part of a wall surface of the cavity, on an opposite side from a cylinder head with respect to an opening edge of the cavity.
With the combustion chamber structure of JP2010-121483A, the fuel spray is injected to collide against a part of the wall surface of the cavity on the opposite side from the cylinder head with respect to the opening edge first, directed downward and then toward the center from the outer circumferential side along the wall surface of the cavity, and further directed toward the fuel injection device. Thus, the fuel can effectively be mixed with air.
Here, by providing the reentrant cavity as described above, the mixing of the fuel with air is stimulated, hazardous combustion products (NOx, soot (i.e., smoke)) can be reduced, and fuel consumption can be improved. Specifically, in a diesel engine in which the reentrant cavity is formed in the piston, when a comparatively large amount of fuel is injected from a fuel injector within a medium or high engine load range, spray of the fuel flows to a circumferential edge portion of the cavity and a flow of the spray reverses along a wall surface of the cavity (changes the direction toward the center of the cavity), namely a tumble flow, occurs. Thus, the mixing of the fuel with air is stimulated. Therefore, the hazardous combustion products (NOx, soot) are reduced while improving the fuel consumption.
Here, the reduction effect against the hazardous combustion products (i.e., the mixing effect of the fuel and air) becomes greater as the volume of the cavity is larger and the tumble flow produced within the cavity becomes stronger.
On the other hand, after the piston reaches a top dead center, a combustion chamber volume is increased as the piston descends. Here, a flow of combustion gas occurs within the combustion chamber to lead the combustion gas outward of the cavity in a radial direction thereof where an increase ratio of the combustion chamber volume is large. Thus, heat of the combustion gas is transmitted to a periphery of a lip portion and causes cooling loss. The cooling loss becomes larger as the combustion chamber volume on the radially outward side of the cavity at the top dead center becomes smaller (i.e., as the cavity becomes larger). Specifically, since the combustion chamber volume on the radially outward side of the cavity has to become smaller as the cavity becomes larger, the increase ratio of the combustion chamber volume on the radially outward side of the cavity becomes large, the flow of the gas within this outward section becomes stronger, and the cooling loss becomes greater.
Therefore, to reduce the generation of the hazardous combustion products while reducing the cooling loss and also improving the fuel consumption, it becomes a challenge to design the cavity of the combustion chamber and the combustion chamber volume on the outward side of the cavity suitably.